Feed horns for reflector dishes

ABSTRACT

A feed horn for a parabolic reflector includes a feed waveguide capable of supporting a first electromagnetic mode and a cylindrical member surrounding an open-ended radiating portion of said feed guide and located about said open end at a distance to cause a second mode to be supported. Said first and second modes as propagating combine to provide a radiation pattern for illuminating said reflector to provide a high performance antenna system.

United States Patent [191 Lewis et a1.

1 June 4, 1974 1 1 FEED HORNS FOR REFLECTOR DISHES [75] lnventors: Robert F. Lewis, Lincroft; James W.

Nelson, Edison, both of NJ.

[73] Assignee: Prodelin, lnc., Hightstown, NJ.

[22] Filed: Apr. 16, 1973 [21] Appl. No.: 351,245

[52] US. Cl 343/775, 343/781, 343/786 [51] Int. Cl.... HOlq 13/06, HOlq 19/14, H0lp1/16 [58] Field of Search 343/786, 781, 772, 775;

[56] References Cited UNITED STATES PATENTS 3,305,870 2/1967 Webb 343/786 3,324,423 6/1967 Webb 343/786 X 3,413,641 1 H1968 Turrin 343/781 3,413,642 11/1968 Cook 343/781 OTHER PUBLICATIONS I Reed, Jr.-Communication Satellite Ground Station Antennas in"The Microwave Journal June 1967; pp. 63-69.

Primary Examiner-James W. Lawrence Assistant Examiner-Marvin Nussbaum Attorney, Agent, or FirmArthur L. Plevy [5 7] ABSTRACT A feed born for a parabolic reflector includes a feed waveguide capable of supporting a first electromagnetic mode and a cylindrical member surrounding an open-ended radiating portion of said feed guide and located about said open end at a distance to cause a second mode to be supported. Said first and second modes as propagating combine to provide a radiation pattern for illuminating said reflector to provide a high performance antenna system.

10 Claims, 3 Drawing Figures i4 1* r0 mm/r 4%12753 g? 25 F is. 1.

, IIIIIIIIIIII mmmm 4mm $815139 24 r0 Mwr I IIIIIIIII A M/D BAND mm (61756172) A 4.36 (/77 0/? F/ELD PATTERN (y PATTERAQ /I 0. 357 L I 5- x /4 A FEED HORNS FOR REFLECTOR DISHES BACKGROUND OF INVENTION phase and amplitude distribution over the surface of the paraboloid or the so called dish without any spillover. When one states without any spillover, it is meant that all energy illuminates only the reflector, as spillover is energy lost by radiation from the feed not incident on the reflector.

If in fact one could provide an electrically invisible structure supporting the feed, an electrically invisible radome and no ohmic losses in the system, the ideal parabolic would be 100 percent efficient or the aperture would provide the full theoretical gain.

The factors that are prevalent in the real world and serve to reduce efficiency are those that cause the antenna to depart from an aperture that is uniformly illuminated in phase and amplitude. These include losses of energy due to spillover, cross-polarized energy, radome and ohmic losses and so on.

In any event, it is known that the feed system for a given paraboloid reflector is greatly responsible for antenna efficiency. Presently, there are a number of general feed systems, all using a main paraboloid reflector. Some of these are the horn feed, Cassegrain feed, Scalar feed, array feed and the shaped reflector feed. Of all of the feed systems, the least efficient is the horn feed and the most efficient is the shaped reflector system.

Due to economical consideration, the horn feed is the most widely used. A horn feed is usually placed at the focal point of the paraboloidal dish. The parabolic antenna with the horn feed has been used over a reasonable range of frequencies with a very low VSWR. One can obtain a variety of polarizations and the feed is relatively insensitive to reflector or dish size.

Efficiency is a concern but even more important are the operating characteristics of the antenna system. Coupled with good efficiency is the important requirement'thatlow side lobes are obtained. It is known that to obtain low side lobe performance, one usually has to pay in efficiency. Similarly, a very high front-to-back ratio would affect the high edge illumination of an antenna. Such operating characteristics are of as much concern as efficiency in that they determine overall performance.

Hence, the prior art has attempted to comply with such desires by providing an additional reflector for a horn feed, wherein the reflector surrounds the horn but is spaced rearwardly from the mouth of the horn. An example of this technique and associated structure is shown in US. Pat. No. 3,553,707 entitled WIDE- BEAM HORN FEED FOR PARABOLIC ANTEN- NAS, issued on Jan. 5, 1971 to R. F. H. Young, et al., and assigned to the Andrew Corporation.

This patent shows an annular reflector plate with a horn feed, which plate is withdrawn from theend of the horn desireably about one-half wavelength, and is isolated from the external surface of the horn by a coaxial choke.

The antenna feed shown in the reference serves to provide a relatively uniform radiation pattern for a paraboloid reflector with suppressed current flow on the outer surface.

In any event, it is desireable to provide a horn feed arrangement for a parabolic reflector which results in a parabolic antenna arrangement having low side lobe with good efficiency, and which feed arrangement is relatively inexpensive, reliable and capable of being adapted to a wide range of frequencies due to an understanding of the operation and a control of amplitude and phase characteristics.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT opened top end and a closed bottom end, said bottom end having an aperture for accommodating said guide to cause the sidewall of said cylindrical member to surround a predesiredportion of said guide at said radiating end to cause a second mode to propagate, whereby said first and second modes combine to provide a desired radiation pattern for said parabolic reflector,

which pattern has sharp cut-off at the parabolic edge which permits low side lobe levels in the main aperture.

BRIEF DESCRIPTION OF FIGURES FIG. 1 is a cross sectional view of a parabolic reflector with a side view of a horn feed according to this invention.

FIG. 2 is a cross sectional side view of an improved horn feed according to this invention.

FIG. 3 is an electric field pattern showing one-half of the field provided by the horn of FIG. 2.

DETAILED DESCRIPTION OF FIGURES I Referring to FIG.-l there is shown a side view of an antenna system employing a paraboloid reflector l0 and a horn feed arrangement 11 according to this invention.

For purposes of the following specification, it will be assumed that the paraboloid is of a true parabolic contour. However, it is realized that any deviation from a true contour may cause loss. Such deviation may be due to errors in fabrication, distortion due to wind forces and translation of the horn feed 11 to other than the focal point.

Generally, it is desireable to utilize a horn feed or Y born 11 in which the pattern of both the E and H plane plitude and phase in the aperture reference plane of the parabola is constant. If one delves into the physical configuration, it would be predicted that energies off axis should be slightly stronger than those on axis. This is due to the distance the wave has to travel to the outside periphery of the dish or parabolic plate with respect to the energy on axis to the center of the dish. It is for this reason that the design of the E and l-l'plane shall have a configuration such that the off axis is slightly greater in energy level than the on axis level thus requiring a slight dip in the primary feed pattern for both the E and H plane.

Modern day communication techniques dictate that the antenna system must also be capable of operating at at least two frequencies. Thus, there is shown dual waveguide inputs l2 and i3 for introducing energy into the horn feed 11'. This feature requires dual polarization capability for two frequencies.

Referring to FIG. 2 there is shown a cross sectional view of the improved horn feed according to this invention. Numeral references a circular waveguide which is used as a feed system. To obtain symmetrical E and H plane patterns, a circular waveguide offers the best possibility. The circular waveguide also helps to minimize cross polarization components. The waveguide 20 is located within a sleeve 21 also of a circular cross sectional configuration to permit the guide 20 to be'positionally moved to the right, left or as desired (shown by double ended arrow). The waveguide 20 has an open front end 22. In an open-ended circular waveship should be such that there are two alternate phases,

for example, two in phase and one out of phase or vice versa. By controlling the out-of-phase versus in-phase intensities, it is thus possible to control the pattern configuration.

As indicated, the circular waveguide 20 provides the TE mode which is non-controllable. It is known that if the system can provide a discontinuity, then other modes can exist simultaneously. Thus there is shown a cylinder 24 surrounding the waveguide 20. The waveguide 20 is shown extending into and surrounded by cylinder 24, a predetermined distance designated as C. The cylinder 24 is shown to be at a length of C plus D and an imaginary plane 25 is shown between C and D and is designatedas a reference plane.

Briefly, the cylindrical member 24 has a back plate 26 with an aperture 28 substantially congruent with the cross section of waveguide 20 to accommodate the same. The back plate 26 supports the cylindrical portion 24 as shown. Alternatively, the back plate 26 and sidewalls 24 could be contiguous.

In any event, it is clear that the circular feed guide 20' is completely surrounded by the cylindrical member 24 and is thereby located within the hollow confines of member 24. The diameter of cylinder 24 is important as compared to the diameter of waveguide 20. The TE mode should have a high impedance in the plane across the aperture of the main feed guide. To accomplish this, the back plate 26 of member 24 is made approximately A-guide wavelength from the open end of the main guide 20.

Upon an investigation of the various TE modes, (N positive integer) it was found that a TE mode could be generated and desireably supported. The ratio of the outer cylinder 24 to the surrounded portion of the waveguide or particularly the dimension C is selected as follows. The dimension C is selected so that it is approximately A guide wavelength at the supported frequency for the TE or the dominant circular waveguide mode. The same dimension C is also approxi-,

mately one-half a guide wavelength at the TE mode. This causes currents due to the TE mode as emanating at the reference plane 25 to be directed back or reflected back off wall 26 and thence combine at the reference plane 25 as they appear in-phase. In a similar manner, current due to the TE mode as reflected back will cancel at plane 25. This causes the E and H planes of the electromagnetic wave as propagating from the horn feed to be symmetrical and to be kept in quadrature. Utilizing this feature, one can now adjust the dimension C, and therefore the ratio of C and D to C, and hence affect the pattern. Thus by controlling the 2 length of the outer cylinder to the surrounded cylinpect of the antenna system with particular concentration of the horn feed'assembly. The final parabolic antenna using this feed arrangement has low side lobe content thus permitting its use in telephony microwave repeaters and other systems.

In order to accommodate an antenna system with the desireable operating characteristics as above described, a two mode configuration was utilized. As explained, the main circular feed guide 20 supports the dominant TE By surrounding the main circular waveguide 20 with an external cylindrical section 24 located with a bottom end to the rear of the main dominant mode guide, it is possible, due to the discontinuity at the open end of the dominant mode, to create high order modes.

- While the above description concentrated on the generation of the TB mode, it was determined that a TM mode could also be generated. However, the generation of the TM mode is such that cancellation appears in the aperture of the large outer cylinder. Even though the TM can be generated, the field configuration is such that it nearly cancels at the aperture of the large cylindrical section and is therefore of small consequence.

Referring to FIG. 3 there is shown the electric lines for a typical TE mode. In the figure the inner dotted circle 30 is the TB dominant mode. It is understood that the field structure within the dotted circle 30 is essentially the same as would be without the outer cylinder 24 of FIG. 2. By varying the position of the inner guide (C) and outer cylinder, the radiation pattern of the feed horn is changed. As one can see, the outer circle B represents the diameter of the cylinder 24 (FIG. 2) while the inner circle A represents the radius of waveguide 30 (FIG. 2). Physically, A 0.685 inches and B 2.l87 inches for a frequency operation between 5.925 to 6.425Gl-IZ. This pattern in essence would be the same charge distribution over the surface of the parabola to be illuminated. It is noted that the peripherial charge lines as S and 51 are small and confined within the circle 40. This is indicative of low side lobe operation and is extremely desireable.

The far outside lobes are due primarily to the electrical charge distribution around the edge of the large outer cylindrical section. In referring to FIG. 3, it will be noted that by the incorporation of the TE mode, the charge distribution has been reduced on the outer periphery of the large circular section. The electrical charges existing on the edges of both the inner and outer circular sections fall in the category of a diffraction problem. The diffraction problem is in general independent of the main radiation pattern. It is, however, well known that the diffraction problem is one of the causes for the existence of side lobe levels at angles from to 1 10. It is for this reason that a series of angular rings forming chokes can be placed around the outer periphery of the large cylindrical section 24. It is obvious that the side lobe levels from the 10 off axis to approximately 110 can be controlled and reduced to a low level by judiciously adjusting the length of the outer circular section. However, the outer cylindrical section should not be reduced to the plane of the main cylindrical feed due to the very stringent characteristics demanded of a high performance antenna system. It is also necessary to make those areas of high charge density such that the charge density is minimized by feathering or tapering in particular the main circular feed system carrying the TE mode. Thus, the main concept of this type of configuration is such that the energy levels off axis of the primary feed can be higher than the on-axis level. This is to obtain as high efficiency as possible and yet maintain side lobe levels within the desired specification.

As stated above, the side lobe levels are dominated by the primary feed pattern, as it impinges at the edge of the parabola. Also, due to the very stringent side lobe level characteristics in anglesfrom 10 to 110, it is important to control the diffraction of the electrical charges at the extremities of the tubular sections. The outer section, being of larger dimension, will have reduced charges by virtue of its size.

Thus, it has been shown how a discontinuity at the open end of the circular waveguide represented by a cylinder within a cylinder can support two modes, both of which combine to control tye type of radiation pattern for antenna systems.

A parabolic antenna operated over a frequency range of 5.925 to 6.425GHZ using dish reflectors of about 6 to 15 feet in diameter with a horn feed (FIG. 2) whereby cylinder 24 was about 4% inches in diameter. Dimension C was approximately between 12 and 1% inches and dimension D was approximately onesixteenth of an inch or less.

The antenna exhibited lower side lobe energy at all angles of concern than prior art parabolic antenna with conventional horn feeds.

We claim:

1. In an antenna system of the type having a reflector dish and a feed system for radiating energy at said-reflector dish, the improvement therewith of a feed system for illuminating said dish while further providing low side lobe content, comprising:

a. an open-ended waveguide capable of supporting a predesired first electromagnetic mode, including means coupled thereto adapted for application to said waveguide of at least a given frequency, and

b. a cylindrical member having an open top end and a closed bottom end, said bottom end having an aperture substantially congruent with said cross section of said open-ended waveguide, said cylindrical member positioned with said aperture encircling said open-ended waveguide at a predetermined distance from said open end, whereby said sidewalls of said cylindrical member extend beyond said open end of said waveguide, said predetermined distance selected such that the length of said openended waveguide, as surrounded by said cylindrical member, is sufficient to cause a second electromagnetic mode to propagate whereby said first mode and said second mode interact to provide at the output a radiation pattern capable of efficiently illuminating said dish for an antenna system, said portion of said cylindrical member not surrounding said waveguide having smooth edges about said open top end and a length substantially less than said predetermined length to cause a low charge density on said surfaces about said open top end to further provide a relatively low side lobe characteristic.

2. The antenna system according to claim 1 wherein:

5. The antenna system according to claim I wherein at least a portion of said open-ended waveguide,- as surrounded by said cylindrical member, is thinner than any portion not so surrounded.

6. The antenna system according to claim 1 wherein said reflector dish is a parabolic dish.

7. A horn feed for a parabolic reflector, comprising:v

a. a feed waveguide having an open radiating end, said guide'capable of supporting a predetermined electromagnetic mode, and t b. a cylindrical member having an open top end and a closed bottom end, said bottom end having an aperture dimensioned to be relatively congruent with the cross section of said waveguide to accommodate said waveguide in a manner to surround said radiating end at a predetennined distance therefrom, to cause a second mode to propagate whereby said first and second modes combine to provide a radiation pattern for said parabolic reflector, said portion of said cylindrical member not surrounding said waveguide having smooth edges about said open top end and a length substantially less than said predetennined length to cause a low charge density on said surfaces about said open top end to further provide an antenna system having a low side lobe content.

8. The horn feed according to claim 7 wherein said feed waveguide is a circular waveguide capable of supporting a TE mode.

9. The horn feed according to claim 8 wherein said second mode is a TE mode.

10. The horn feed according to claim 7 wherein said 5 predetermined distance is electrically twice the distance for said secondmode as compared to said first 

1. In an antenna system of the type having a reflector dish and a feed system for radiating energy at said reflector dish, the improvement therewith of a feed system for illuminating said dish while further providing low side lobe content, comprising: a. an open-ended waveguide capable of supporting a predesired first electromagnetic mode, including means coupled thereto adapted for application to said waveguide of at least a given frequency, and b. a cylindrical member having an open top end and a closed bottom end, said bottom end having an aperture substantially congruent with said cross section of said open-ended waveguide, said cylindrical member positioned with said aperture encircling said open-ended waveguide at a predetermined distance from said open end, whereby said sidewalls of said cylindrical member extend beyond said open end of said waveguide, said predetermined distance selected such that the length of said open-ended waveguide, as surrounded by said cylindrical member, is sufficient to cause a second electromagnetic mode to propagate whereby said first mode and said second mode interact to provide at the output a radiation pattern capable of efficiently illuminating said dish for an antenna system, said portion of said cylindrical member not surrounding said waveguide having smooth edges about said open top end and a length substantially less than said predetermined length to cause a low charge density on said surfaces about said open top end to further provide a relatively low side lobe characteristic.
 2. The antenna system according to claim 1 wherein: a. said open-ended waveguide is a circular waveguide and said first predesired electromagnetic mode is the TE11 mode.
 3. The antenna system according to claim 2 wherein said second electromagnetic mode is a TE12 mode.
 4. The antenna system according to claim 2 wherein said predetermineD distance is equal to one-fourth of a guide wavelength at said given frequency for said TE11 mode.
 5. The antenna system according to claim 1 wherein at least a portion of said open-ended waveguide, as surrounded by said cylindrical member, is thinner than any portion not so surrounded.
 6. The antenna system according to claim 1 wherein said reflector dish is a parabolic dish.
 7. A horn feed for a parabolic reflector, comprising: a. a feed waveguide having an open radiating end, said guide capable of supporting a predetermined electromagnetic mode, and b. a cylindrical member having an open top end and a closed bottom end, said bottom end having an aperture dimensioned to be relatively congruent with the cross section of said waveguide to accommodate said waveguide in a manner to surround said radiating end at a predetermined distance therefrom, to cause a second mode to propagate whereby said first and second modes combine to provide a radiation pattern for said parabolic reflector, said portion of said cylindrical member not surrounding said waveguide having smooth edges about said open top end and a length substantially less than said predetermined length to cause a low charge density on said surfaces about said open top end to further provide an antenna system having a low side lobe content.
 8. The horn feed according to claim 7 wherein said feed waveguide is a circular waveguide capable of supporting a TE11 mode.
 9. The horn feed according to claim 8 wherein said second mode is a TE12 mode.
 10. The horn feed according to claim 7 wherein said predetermined distance is electrically twice the distance for said second mode as compared to said first mode. 